packet-inject: fix injection outside of netns
packet-inject: fix injection outside of netns

Closes: #409

lebauce/skydive,skydive-project/skydive,skydive-project/skydive,safchain/skydive,safchain/skydive,safchain/skydive,skydive-project/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,safchain/skydive,skydive-project/skydive,lebauce/skydive,lebauce/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive,lebauce/skydive,safchain/skydive,safchain/skydive,skydive-project/skydive